1. Field of the Invention
This invention relates to alarm systems and, more particularly, to a multi-zone alarm system for the detection and indication of an alarm condition in variously identified zones. Specifically, the present invention relates to an improved alarm system which monitors multiple zones with normally closed circuit sensing devices in a series connected circuit by means of a single pair wire.
2. Description of the Prior Art
In alarm systems employed to sense intrusion, fire or other conditions, techniques are known for the determination at a central location of the remote zone in which an alarm has occurred. Examples of such systems are illustrated in U.S. Pat. No. 4,274,087, No. 4,625,198 and No. 4,728,946. In such systems, a communications path is a generally established between each remote alarm sensor and a central location, the communications path being provided by means of a separate communications line from the central location to each remote station, or by use of a common communications line and multiplexed signaling techniques, such as time division multiplexing or frequency division multiplexing.
It is advantageous to employ a two-wire communications path forming a single alarm loop in which all alarm sensors are connected. Such a single loop can minimize the amount of wiring necessary to interconnect the central location with the remote sensors and can provide relatively simple and efficient connection of the remote sensors with the central location. It is typically required in an alarm system to provide the capability of identifying each sensor or each zone in which an alarm has occurred, and an example of this type of system is disclosed in U.S. Pat. No. 4,423,410. However, there remains quite a few drawbacks and limitations to existing two-wire systems available on the market including the ability to function both in the armed and disarmed modes as well as to indicate when the system has been cut during its disarmed mode.